Expandable thermoplastic beads, e.g. polyvinylidene chloride-- acrylonitrile, polystyrene or styrene-acrylonitrile beads-- have been produced commercially in a suspension polymerization process where the liquid monomer is dispersed in an aqueous medium containing one or several suspension agents, a hydrocarbon blowing agent and a polymerization initiator.
In the initial stage of the polymerization the monomers and the blowing agent form a droplet with only one phase. In a later stage of the polymerization the blowing agent is not soluble in the polymer phase and builds a separate phase in the form of small inclusions in the polymer droplet.
The obtained beads comprise polymer shells containing the liquid, volatile blowing agent. The beads expand by being heated to a temperature above the boiling point of the blowing agent and above the softening point of the polymer, e.g. at about 70.degree. C. for polyvinylidene chloride-acrylonitrile beads. The beads can be used for production of cellular material, which finds many uses in e.g. the insulation and packaging industries.
A serious problem in all polymerization techniques is the residual monomer content. The monomers are more or less poisonous and since the polymerization can never be conducted to a 100% conversion, both the resulting polymerizate and process water is contaminated with residual monomers. This problem is especially accentuated in the polymerization of expandable beads as these also contain a third phase, i.e. the blowing agent, in which monomers may be dissolved.
Large amounts of acrylonitrile in copolymers (e.g. polyvinylidene chloride-acrylonitrile and styrene-acrylonitrile copolymers) are drastically restricted in their area of application and their market since free acrylonitrile in the process water and residual acrylonitrile in the beads increase the health hazards for the people handling the beads.
In recent years numerous methods for purifying polymerizates from residual monomer have been suggested (so called "stripping"), especially in the polyvinyl chloride technology. The most common methods relate to the use of elevated temperatures in order to cause diffusion of the monomers from the polymer phase. The elevated temperature increases the mobility of the monomer molecules and plasticizes the polymer and both these factors increases the rate of stripping.
Today the most commonly used method for stripping vinyl chloride from polyvinyl chloride is to treat the resulting polymer and process water with steam at temperatures within the range of 80- 125.degree. C. at ambient pressure. In certain processes the stripping is conducted at reduced pressure. Such a process is described in e.g. DT-OS No. 25 21 780.
It is not possible to use these methods in stripping monomers from expandable beads, as these are very sensitive to heat and pressure. At higher temperatures, as well as at lower pressure, the beads will expand depending on the blowing agent. Another problem is that the monomers in expandable beads, and especially acrylonitrile, are more water-soluble than vinyl chloride, and are thus more difficult to remove from the water phase. Moreover many of the commonly used monomers in the production of expandable beads have a high boiling point which makes them difficult to remove from the water phase by raising the temperature. Thus acrylonitrile has a boiling point of 77.degree. C., styrene 145.degree. C. and vinylidene chloride 32.degree. C. while vinyl chloride has a boiling point of -14.degree. C.
The high concentration of blowing agent inside the beads and the high temperature required for stripping produce a considerable pressure inside the beads and they expand in the unfilled or gas-filled part of the stripping device.
The pre-expanded beads are a problem and increase the cost of production. The pre-expanded beads have to be separated by filtration or by other means from unexpanded beads, and the pre-expanded beads increase the amount of waste.
Attempts have been made to avoid the pre-expansion of the beads by applying a high pressure, e.g. 500- 1500 kPa of an inert gas, e.g. nitrogen, during stripping. However, this does not eliminate the formation of pre-expanded beads. The gas does not actually apply a higher pressure on the outside of the beads. An equilibrium between the gas inside the beads and the gas in the "free volume" in the stripping device is established. The beads continue to expand because of the "free volume" and the high pressure of hydrocarbon inside the beads.